From 8daae614204608cdfe11566bf0a471152a7bb1ee Mon Sep 17 00:00:00 2001
From: David Hunter <reverendgumby@gmail.com>
Date: Tue, 19 May 2026 22:00:42 -0700
Subject: [PATCH] adpcm_dec.py: HuC6230 ADPCM decoder

---
 rtl/tb/adpcm_dec.py | 56 +++++++++++++++++++++++++++++++++++++++++++++
 1 file changed, 56 insertions(+)
 create mode 100644 rtl/tb/adpcm_dec.py

diff --git a/rtl/tb/adpcm_dec.py b/rtl/tb/adpcm_dec.py
new file mode 100644
index 0000000..dc5f986
--- /dev/null
+++ b/rtl/tb/adpcm_dec.py
@@ -0,0 +1,56 @@
+# HuC6230 ADPCM decoder
+
+# Input format: 16-bit (as read from KRAM), big-endian
+# Output format: 16-bit signed PCM, big-endian
+
+import sys
+
+ltab = [-1, -1, -1, -1, 2, 4, 6, 8]
+
+stab = [
+    16, 17, 19, 21, 23, 25, 28, 31,
+    34, 37, 41, 45, 50, 55, 60, 66,
+    73, 80, 88, 97, 107, 118, 130, 143,
+    157, 173, 190, 209, 230, 253, 279, 307,
+    337, 371, 408, 449, 494, 544, 598, 658,
+    724, 796, 876, 963, 1060, 1166, 1282, 1411,
+    1552
+]
+
+p = 0x20000
+sl = 0
+ss = stab[sl]
+
+def decode(a, fout):
+    global p, sl, ss
+
+    c = a >> 3  # code: step sign
+    d = a & 7   # data: step magnitude
+
+    dt = ((d + 1) * 8) * ss
+    dt = -dt if c else dt
+    p = p + dt
+    p = max(min(p, 0x3ffff), 0)
+
+    so = ((p >> 2) - 0x8000) & 0xffff
+    fout.write(bytes([so >> 8, so & 0xff]))
+
+    sl = sl + ltab[d]
+    sl = max(min(sl, 48), 0)
+    ss = stab[sl]
+
+def main():
+    fin = open(sys.argv[1], "rb")
+    fout = open(sys.argv[2], "wb")
+
+    while True:
+        w = fin.read(2)
+        if len(w) < 2:
+            break
+        decode((w[1] >> 0) & 0xf, fout)
+        decode((w[1] >> 4) & 0xf, fout)
+        decode((w[0] >> 0) & 0xf, fout)
+        decode((w[0] >> 4) & 0xf, fout)
+
+if __name__ == '__main__':
+    main()